A scientific analysis unveiled at the COP30 Local Leaders Forum has revealed a worrying and underappreciated danger in the fight against global warming: nighttime temperatures in major cities are rising far faster than daytime highs—up to 12 times faster in some locations—creating a new wave of health risks across the planet.
The study, titled “Extreme Heat and the Shrinking Diurnal Range: A Global Evaluation of Oppressive Air Mass Character and Frequency,” was released by Climate Resilience for All, and analyzed 30 years of weather data (1994–2024) across 100 major global cities. It focused on two of the most dangerous heat conditions for human health—dry tropical (DT) and moist tropical (MT) air masses—known to drive spikes in heat-related illnesses and mortality.
While climate policy debates have centered on keeping global temperature rise below 1.5°C, the new research shows that minimum nighttime temperatures are increasing much more quickly during extreme heat events, significantly narrowing the gap between daytime and nighttime conditions.
According to the report, 83% of the cities studied are experiencing sustained increases in nighttime heat. This shrinking difference—known as the diurnal temperature range—means millions are losing their only window for relief from oppressive heat.
In Africa, the impacts are especially severe.
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13 out of 15 cities in the study showed higher nighttime temperatures under humid (moist tropical) conditions, and
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10 out of 14 recorded higher nighttime temperatures under dry tropical conditions.
Cities such as Agadir, Morocco, and Cairo, Egypt are among the worst hit. In Agadir, nighttime temperatures under dry tropical conditions are rising 1°C every 5.6 years, while in Cairo, moist tropical nights are warming 1°C every 16.8 years.
Other cities showing marked decreases between daytime and nighttime temperatures include Mombasa (Kenya), Brazzaville (Congo), Bamako (Mali), Abidjan (Côte d’Ivoire), Dakar (Senegal), Cape Town (South Africa), and Reunion Island, among others.
The analysis also found that the number of extreme heat days is increasing sharply:
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Moist tropical weather patterns have surged by nearly 49% across Africa and 37% globally over the past three decades.
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Dry tropical conditions rose 14% in frequency, meaning cities now experience one additional DT day roughly every 3.4 years.
Each rise in DT and MT events effectively displaces cooler, safer weather patterns, reducing respite from heat extremes.
“Before this analysis, we did not know how rapidly nighttime heat has been rising within the most dangerous air masses,” said Larry Kalkstein, Chief Heat Science Advisor at Climate Resilience for All and the study’s lead author. “It is critical for us to understand how the heat of summer—that sends people to the emergency room—is shifting, and what we are overlooking when we talk about it.”
For cities in the Global South, where air conditioning remains a luxury, the findings strike particularly close to home. Mayor Yvonne Aki-Sawyerr of Freetown, Sierra Leone, emphasized:
“This study reflects the lived reality of so many cities in the global south. Most of our residents lack access to cooling, and women market traders facing extreme heat all day return to homes that are dangerously hot at night. As we implement our heat action plan, this study sounds the alarm on the hidden threat of nighttime temperatures and pushes us toward solutions that protect people.”
Kathy Baughman McLeod, CEO of Climate Resilience for All, urged policymakers to act swiftly:
“We want this analysis to mobilize city and health leaders to urgently broaden their view of what is a 24-hour heat crisis. This research uncovers a critical blind spot in our understanding of extreme heat.”
Health experts warn that high nighttime temperatures prevent the body from cooling down, leading to dehydration, cardiovascular strain, and increased mortality—especially among older adults, women, and those in poorly ventilated homes.
The report recommends that governments expand heat warning systems beyond daytime highs to include overnight temperature risks, improve public health communication, and strengthen urban adaptation strategies—such as better housing design, urban greening, and early-warning alerts that account for multi-day, high-intensity heatwaves with little nocturnal relief.
As COP30 continues in Belem, this analysis adds new urgency for cities and nations alike to confront not only how hot the world is getting—but how little it now cools down.